1. Field of the Invention
The present invention relates to a gear tooth sensor and more particularly, to a gear tooth sensor having increased sensitivity and temperature tolerance which can detect relatively small ferrous targets over a relatively wide temperature range for use in relatively compact applications, such as sensing the rotation of an automotive cam shaft drive wheel.
2. Description of the Prior Art
Gear tooth sensors are generally known in the art. An example of such a gear tooth sensor is disclosed in U.S. Pat. No. 4,970,463, assigned to the same assignee as the present invention and hereby incorporated by reference. Such sensors normally include a magnetic flux responsive element, such as a Hall effect element, a magnet and may include one or more flux concentrators, all rigidly disposed adjacent one another. The configuration of such a sensor as in the '463 patent is such that an axis perpendicular to the opposing north and south pole faces of the magnet, hereinafter called a "magnetic axis", is generally perpendicular to the path of travel of the gear tooth on the ferrous target wheel at an instant when the target is adjacent the magnet. With such a configuration, the sensitivity of the sensor is relatively low, requiring relatively large ferrous targets for proper operation, which are generally not suitable in various compact applications, such as the sensing of rotation of an automotive cam shaft drive wheel. More particularly, in relatively compact applications, the distance between gullets (i.e. distance between adjacent teeth on a ferrous target wheel) is often limited to, for example, generally around 6 millimeters (mm). The sensor as disclosed in the '463 patent is not capable of sensing ferrous targets with such spacing between gullets on the ferrous target wheel.
In such compact applications, other known gear tooth sensors normally use a differential input magnetic flux sensitive element, such as an Allegro model No. 3056 integrated circuit (IC). Such differential IC's are normally about twice the size of a single input IC, such as Allegro model Nos. 3131 or 3134 single input IC's, normally used with gear tooth sensors of the type disclosed in the '463 patent. Thus, the cost of a differential IC is significantly higher than a single input IC.
In addition to being more expensive, there are problems with sensors which utilize such differential Hall effect IC's. For example, the Allegro model No. 3056 is normally formed on a relatively large die with two Hall effect sensitive arrays disposed adjacent to opposing ends of the die, for example, spaced 2 mm apart from center line to center line. The differential IC's use two (2) Hall effect elements which can experience about twice the temperature drift errors as an IC with a single Hall effect element.
There are other problems with known gear tooth sensors. In particular, known gear tooth sensors normally operate at a relatively large tolerance relative to the physical position of the gear tooth. For example, in an application where the sensor is used to indicate the passage of a target edge, such known sensors normally will not operate exactly at the target edge, but perhaps several degrees before or after the passing of the target edge; for example, .+-.2.degree.. In many applications, such a relatively wide tolerance is unacceptable. In some applications, such as a cam shaft sensor, relatively narrower tolerances may be required, such as +or -1.degree.. In such applications, such known gear tooth sensors would not be useful.
Another known problem with gear tooth sensors is that each gear tooth sensor is normally configured for a single application and is not adapted to be used in any other application. For example, as discussed above, the gear tooth sensor disclosed in the '463 patent is adapted to be used in only one application where the magnetic axis is generally perpendicular to the direction of travel of the ferrous target. The gear tooth sensor disclosed in the '463 patent is not adapted to be used in other applications, such as in applications where, due to the space constraints and specific geometry of the application in which the sensor is to be used, the magnetic axis would need to be generally parallel to the direction of travel of the ferrous target. As such, various known gear tooth sensors, such as the sensor disclosed in the '463 patent, have only limited utility. Thus, various customers and original equipment manufacturers (OEM) are required to purchase different sensors for each of their various applications.